How to Set VFD Skip Frequencies to Avoid Resonance

Back to Blog

Find the Resonant Bands by Sweeping the Range

You cannot skip a band you have not located, so start by mapping where the machine shakes. With the machine safe to run across its range, slowly increase the frequency from minimum toward maximum, pausing at intervals, and watch a vibration reading or feel and listen for the machine to get rough. A resonance announces itself as a speed where vibration climbs sharply and then falls again as you pass through. Note the frequency at the peak and the band around it where the roughness is noticeable.

Sweep slowly and in both directions if you can, because a resonance can be sharp and easy to step over with a coarse sweep. Move up through the range and then back down, confirming the same bad speeds appear both ways. The concept behind why a machine has these speeds, and how the drive normally avoids them, is described in the page on VFD skip-frequency avoidance, which frames what you are hunting for during the sweep.

Distinguish a genuine structural resonance from another vibration source. A rough spot at one specific speed that comes and goes as you pass through is a resonance; a vibration that grows steadily with speed or is present everywhere points to a balance, alignment, or bearing problem instead, which skip frequencies will not fix. Where the vibration signature is ambiguous, condition-monitoring analysis such as that in the page on motor current signature analysis helps separate a resonance to skip from a mechanical fault to repair.

Configure the Skip Windows and Their Width

For each bad frequency you found, set a skip band centered on it. The drive lets you define one or more skip frequencies, each with a width or a band around a center point, and it will refuse to run steadily inside that window. Center the band on the peak vibration frequency and make it wide enough to cover the whole range where the machine was rough, so no part of the resonance is left inside the allowed range.

Choose the width deliberately, not too narrow and not too wide. Too narrow a band leaves the edges of the resonance still runnable, so the machine can settle just inside the rough zone; too wide a band needlessly removes usable speeds from the operating range, which can matter if the process wants to run near there. Set the width to cover the rough range plus a small margin on each side, so the machine reliably steps clear of the resonance without giving up more speed range than necessary.

Understand how the drive treats the band during ramps. The drive passes through a skip band while accelerating or decelerating, because it must cross the speed to reach the other side, but it will not hold steady inside it, jumping instead to the nearer edge if a command lands within. This means a commanded speed inside a skip window produces one of the band edges, which interacts with a process PID or analog reference, so confirm the control scheme tolerates the machine snapping to a band edge rather than sitting exactly where commanded.

Verify the Machine Never Dwells in a Rough Spot

Prove each band by commanding a speed inside it. Set a reference in the middle of a skip window and confirm the drive runs at the band edge instead, not inside the window. Do this for every band you configured, because a skip frequency that was entered wrong, or a width that is too small, lets the machine settle in the rough zone anyway. The point of the feature is that no steady operating speed lands in a resonance, so test that directly.

Re-sweep the range to confirm the machine is smooth everywhere it now runs steadily. Walk the frequency across the operating band again and verify that every speed the drive will hold is quiet, with the rough zones now unreachable as steady setpoints. If a new rough spot appears that you missed the first time, add a band for it. The commissioning is complete when the entire runnable range is smooth and only the transient pass-through during ramps crosses a resonance.

Document the bands and watch them over time, because resonances can shift. A change in mounting, a loosened foundation bolt, added piping, or bearing wear can move or create a resonance, so a machine that was smooth at commissioning can develop a new rough band later. Because vibration and running frequency are values a monitoring system can trend, a rising vibration at a speed that used to be fine is an early sign the mechanical condition has changed, prompting a re-sweep and possibly a new or widened skip band.

Frequently Asked Questions